Black stool is most often caused by something you ate, drank, or took as a supplement or medication. Iron pills, bismuth-based stomach remedies, black licorice, blueberries, and activated charcoal can all turn your stool noticeably dark. Less commonly, black stool signals bleeding in the upper digestive tract, which produces a distinctive tarry, sticky, foul-smelling stool called melena. The difference between a harmless color change and a medical emergency comes down to texture, smell, and accompanying symptoms.
Foods and Supplements That Turn Stool Black
Several everyday items can darken your stool enough to cause alarm. Iron supplements are one of the most common culprits. Iron reacts with digestive enzymes in your gut, turning stool dark green to black. The texture tends to be firm or slightly sticky, but it won’t have an unusual smell. Blueberries, blood sausage, black licorice, and activated charcoal can produce a similar color shift.
Bismuth, the active ingredient in Pepto-Bismol and similar stomach medicines, causes black stool through a specific chemical reaction. Bismuth combines with trace amounts of sulfur in your saliva and digestive system to form bismuth sulfide, a black-colored compound. This same reaction can also temporarily darken your tongue. It’s completely harmless.
The reassuring thing about all of these causes is how quickly the color returns to normal once you stop. After discontinuing iron supplements or bismuth medications, stool typically goes back to its usual brown within 48 to 72 hours.
When Black Stool Means Bleeding
Black stool caused by bleeding in the upper digestive tract (the esophagus, stomach, or first part of the small intestine) is called melena. It takes at least 50 milliliters of blood in the stomach to turn stool black, roughly the equivalent of a few tablespoons. The first dark stool typically appears 4 to 20 hours after the bleeding begins, and a significant bleed can produce tarry stools for up to five days.
The most common medical cause is a peptic ulcer, a sore in the stomach lining or the upper small intestine. The two main drivers of peptic ulcers are infection with a bacterium called H. pylori and long-term use of anti-inflammatory painkillers like ibuprofen, naproxen, or aspirin. Your risk goes up if you take high doses of these painkillers, use more than one at the same time, or combine them with other medications that irritate the stomach lining.
Another cause is ruptured esophageal varices, which are swollen veins in the esophagus that develop in people with liver cirrhosis. When these veins break open, even a small amount of bleeding can produce dark streaks in the stool. Larger bleeds cause fully black, tarry stools and can be life-threatening.
How to Tell the Difference
The physical characteristics of the stool itself are the most reliable clue. Melena is jet black, tarry, and sticky. It clings to the toilet bowl and is difficult to flush. Most distinctively, it has a strong, offensive odor that’s noticeably different from normal stool. This smell comes from blood being broken down by bacteria and enzymes as it moves through the digestive tract. The longer the blood has traveled, the darker and more pungent the stool becomes.
Stool that’s been stained black by food, iron, or bismuth looks dark but doesn’t have that sticky, tar-like consistency. It also doesn’t carry the same distinctive foul smell. If you’re unsure, think about what you’ve eaten or taken in the last day or two. A bowl of blueberries or an iron pill is the most likely explanation.
Symptoms That Signal an Emergency
Black, tarry stool on its own warrants attention, but certain accompanying symptoms indicate active bleeding that needs immediate medical care. These include vomiting blood or material that looks like coffee grounds, feeling lightheaded or dizzy, rapid heartbeat, unusual weakness or fatigue, and abdominal pain. These symptoms suggest your body is losing enough blood to affect circulation.
If you’re taking NSAIDs regularly and notice black stool, don’t dismiss it as a medication side effect. Unlike iron and bismuth, painkillers don’t stain stool black. They cause it by creating ulcers that bleed.
How Doctors Test for Hidden Blood
When stool color is ambiguous, doctors can test for hidden (occult) blood using a stool sample. The newer version of this test, called a fecal immunochemical test (FIT), detects blood with about 86 to 95 percent accuracy depending on the specific test used. It’s significantly more reliable than the older guaiac-based test, which catches only 50 to 70 percent of cases.
These tests aren’t perfect. In people with iron-deficiency anemia, about 42 percent of those who actually had a source of bleeding in their digestive tract got a false-negative result, meaning the test missed it. So a negative stool test doesn’t always rule out bleeding, especially if other symptoms are present. In those cases, doctors typically follow up with an endoscopy to look directly at the lining of the upper digestive tract.
Common Causes at a Glance
- Iron supplements: Dark green to black, firm stool. Resolves 2 to 3 days after stopping.
- Bismuth medications: Black stool and sometimes a black tongue. Harmless, clears quickly after stopping.
- Dark-colored foods: Blueberries, black licorice, blood sausage, activated charcoal. Temporary and harmless.
- Peptic ulcers: Tarry, sticky, foul-smelling stool. Often linked to H. pylori infection or long-term painkiller use.
- Esophageal varices: Associated with liver disease. Can range from dark streaks to fully black, tarry stool depending on severity of bleeding.